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(p200300cbc7092700cdd8dcb02a698783.dip0.t-ipconnect.de. [2003:cb:c709:2700:cdd8:dcb0:2a69:8783]) by smtp.gmail.com with ESMTPSA id a9sm4002911wmm.32.2022.01.26.08.58.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Jan 2022 08:58:41 -0800 (PST) Message-ID: <5a565d5a-0540-4041-ce63-a8fd5d1bb340@redhat.com> Date: Wed, 26 Jan 2022 17:58:41 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [v2 PATCH] fs/proc: task_mmu.c: don't read mapcount for migration entry To: Yang Shi Cc: Jann Horn , "Kirill A. Shutemov" , Matthew Wilcox , Andrew Morton , Linux MM , Linux Kernel Mailing List , stable References: <20220120202805.3369-1-shy828301@gmail.com> <5b4e2c29-8f1a-5a68-d243-a30467cc02d4@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: t89aydiynxueow4r6f6f5rxqetx53j8i X-Rspam-User: nil Authentication-Results: imf03.hostedemail.com; dkim=pass header.d=redhat.com header.s=mimecast20190719 header.b=feaNRKpD; spf=none (imf03.hostedemail.com: domain of david@redhat.com has no SPF policy when checking 170.10.129.124) smtp.mailfrom=david@redhat.com; dmarc=pass (policy=none) header.from=redhat.com X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D790D20003 X-HE-Tag: 1643216325-988256 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 26.01.22 17:53, Yang Shi wrote: > On Wed, Jan 26, 2022 at 3:57 AM David Hildenbrand wrote: >> >> On 26.01.22 12:48, Jann Horn wrote: >>> On Wed, Jan 26, 2022 at 12:38 PM David Hildenbrand wrote: >>>> On 26.01.22 12:29, Jann Horn wrote: >>>>> On Wed, Jan 26, 2022 at 11:51 AM David Hildenbrand wrote: >>>>>> On 20.01.22 21:28, Yang Shi wrote: >>>>>>> The syzbot reported the below BUG: >>>>>>> >>>>>>> kernel BUG at include/linux/page-flags.h:785! >>> [...] >>>>>>> RIP: 0010:PageDoubleMap include/linux/page-flags.h:785 [inline] >>>>>>> RIP: 0010:__page_mapcount+0x2d2/0x350 mm/util.c:744 >>> [...] >>>>>> Does this point at the bigger issue that reading the mapcount without >>>>>> having the page locked is completely unstable? >>>>> >>>>> (See also https://lore.kernel.org/all/CAG48ez0M=iwJu=Q8yUQHD-+eZDg6ZF8QCF86Sb=CN1petP=Y0Q@mail.gmail.com/ >>>>> for context.) >>>> >>>> Thanks for the pointer. >>>> >>>>> >>>>> I'm not sure what you mean by "unstable". Do you mean "the result is >>>>> not guaranteed to still be valid when the call returns", "the result >>>>> might not have ever been valid", or "the call might crash because the >>>>> page's state as a compound page is unstable"? >>>> >>>> A little bit of everything :) >>> [...] >>>>> In case you mean "the result might not have ever been valid": >>>>> Yes, even with this patch applied, in theory concurrent THP splits >>>>> could cause us to count some page mappings twice. Arguably that's not >>>>> entirely correct. >>>> >>>> Yes, the snapshot is not atomic and, thereby, unreliable. That what I >>>> mostly meant as "unstable". >>>> >>>>> >>>>> In case you mean "the call might crash because the page's state as a >>>>> compound page could concurrently change": >>>> >>>> I think that's just a side-product of the snapshot not being "correct", >>>> right? >>> >>> I guess you could see it that way? The way I look at it is that >>> page_mapcount() is designed to return a number that's at least as high >>> as the number of mappings (rarely higher due to races), and using >>> page_mapcount() on an unlocked page is legitimate if you're fine with >>> the rare double-counting of references. In my view, the problem here >>> is: >>> >>> There are different types of references to "struct page" - some of >>> them allow you to call page_mapcount(), some don't. And in particular, >>> get_page() doesn't give you a reference that can be used with >>> page_mapcount(), but locking a (real, non-migration) PTE pointing to >>> the page does give you such a reference. >> >> I assume the point is that as long as the page cannot be unmapped >> because you block it from getting unmapped (PT lock), the compound page >> cannot get split. As long as the page cannot get unmapped from that page >> table you should have at least a mapcount of 1. > > If you mean holding ptl could prevent THP from splitting, then it is > not true since you may be in the middle of THP split just exactly like > the race condition solved by this patch. While you hold the PT lock and discover a mapped page, unmap_page() cannot continue and unmap the page. That's what I meant "as long as the page cannot be unmapped". What doesn't work is if you hold the PT lock and discover a migration entry, because then you're already past unmap_page(). That's the issue you're fixing. > > Just page lock or elevated page refcount could serialize against THP > split AFAIK. > >> >> But yeah, using the mapcount of a page that is not even mapped >> (migration entry) is clearly wrong. >> >> To summarize: reading the mapcount on an unlocked page will easily >> return a wrong result and the result should not be relied upon. reading >> the mapcount of a migration entry is dangerous and certainly wrong. > > Depends on your usecase. Some just want to get a snapshot, just like > smaps, they don't care. Right, but as discussed, even the snapshot might be slightly wrong. That might be just fine for smaps (and I would have enjoyed a comment in the code stating that :) ). -- Thanks, David / dhildenb